Apparatus for polishing thin, flat semiconductor wafers is well known in the art. Such apparatus normally includes a polishing head which carries a membrane for engaging and forcing a semiconductor wafer against a wetted polishing surface, such as a polishing pad. Either the pad, or the polishing head is rotated and oscillates the wafer over the polishing surface. The polishing head is formed downwardly onto the polishing surface by a pressurized air system or, similar arrangement. The downward force pressing the polishing head against the polishing surface can be adjusted as desired. The polishing head is typically mounted on an elongated pivoting carrier arm, which can move the pressure head between several operative positions. In one operative position, the carrier arm positions a wafer mounted on the pressure head in contact with the polishing pad. In order to remove the wafer from contact with the polishing surface, the carrier arm is first pivoted upwardly to lift the pressure head and wafer from the polishing surface. The carrier arm is then pivoted laterally to move the pressure head and wafer carried by the pressure head to an auxiliary wafer processing station. The auxiliary processing station may include, for example, a station for cleaning the wafer and/or polishing head; a wafer unload station; or, a wafer load station.
More recently, chemical-mechanical polishing apparatus has been employed in combination with a pneumatically actuated polishing head. CMP apparatus is used primarily for polishing the front face or device side of a semiconductor wafer during the fabrication of semiconductor devises on the wafer. A wafer is "planarized" or smoothed one or more times during a fabrication process in order for the top surface of the wafer to be as flat as possible. A wafer is polished by being placed on a carrier and pressed face down onto a polishing pad covered with a slurry of colloidal silica or alumina in de-ionized water.
A perspective view of a typical CMP apparatus is shown in FIG. 1A. The CMP apparatus 10 consists of a controlled mini-environment 12 and a control panel section 14. In the controlled mini-environment 12, typically four spindles 16, 18, 20, and 22 are provided (the fourth spindle 22 is not shown in FIG. 1A) which are mounted on a cross-head 24. On the bottom of each spindle, for instance, under the spindle 16, a polishing head 26 is mounted and rotated by a motor (not shown). A substrate such as a wafer is mounted on the polishing head 26 with the surface to be polished mounted in a face-down position (not shown). During a polishing operation, the polishing head 26 is moved longitudinally along the spindle 16 in a linear motion across the surface of a polishing pad 28. As shown in FIG. 1A, the polishing pad 28 is mounted on a polishing disc 30 rotated by a motor (not shown) in a direction opposite to the rotational direction of the polishing head 26.
Also shown in FIG. 1A is a conditioner arm 32 which is equipped with a rotating conditioner disc 34. The conditioner arm 32 pivots on its base 36 for conditioning the polishing pad 38 for the in-situ conditioning of the pad during polishing. While three stations each equipped with a polishing pad 28, 38 and 40 are shown, the fourth station is a head clean load/unload (HCLU) station utilized for the loading and unloading of wafers into and out of the polishing head. After a wafer is mounted into a polishing head in the fourth head cleaning load/unload station, the cross head 24 rotates 90.degree. clockwise to move the wafer just loaded into a polishing position, i.e., over the polishing pad 28. Simultaneously, a polished wafer mounted on spindle 20 is moved into the head clean load/unload station for unloading.
A cross-sectional view of a polishing station 42 is shown in FIGS. 1B and 1C. As shown in FIG. 1B, a rotating polishing head 26 which holds a wafer 44 is pressed onto an oppositely rotating polishing pad 28 mounted on a polishing disc 30 by adhesive means. The polishing pad 28 is pressed against the wafer surface 46 at a predetermined pressure. During polishing, a slurry 48 is dispensed in droplets onto the surface of the polishing pad 28 to effectuate the chemical mechanical removal of materials from the wafer surface 46.
An enlarged cross-sectional representation of the polishing action which results from a combination of chemical and mechanical effects is shown in FIG. 1C. The CMP method can be used to provide a planner surface on dielectric layers, on deep and shallow trenches that are filled with polysilicon or oxide, and on various metal films. A possible mechanism for the CMP process involves the formation of a chemically altered layer at the surface of the material being polished. The layer is mechanically removed from the underlying bulk material. An outer layer is than regrown on the surface while the process is repeated again. For instance, in metal polishing, a metal oxide layer can be formed and removed repeatedly.
Referring now to FIG. 2A, wherein a conventional carousel 50 (or a cross member) for a CMP machine is shown. In carousel 50, four spindle mounts 52,54,56 and 58 are provided in 90.degree. intervals. As illustrated by spindle mount 56, a traversing platform 60 is sued for mounting a spindle (not shown) thereto. The spindle is in turn connected to a polishing head (not shown) for the mounting of a substrate to be polished. It should be noted that carousel 50 is shown in FIG. 2A in an upside-down position. The mounting platform 60 is controlled by motor 62 for traversing on track 64 during a polishing operation. The traversing action of the mounting platform 60 relative to the polishing head (not shown) facilitates an uniform polishing operation on the substrate surface.
Each of the spindle mounts 52,54,56 and 58 must be calibrated prior to the start of a series of CMP processes. They must also be calibrated after each preventive maintenance service during which the polishing head spindle may have been removed. The calibration can be conducted in a load cup 66 shown in FIG. 2B. The load cup 66, also known as a head clean/load/unload station or HCLU, is used for performing major functions of loading, unloading wafers onto, from a polishing head. In a typical CMP apparatus such as one manufactured by Applied Materials, Inc. of Santa Clara, Calif., a single HCLU station is utilized in conjunction with three polishing discs (not shown) which are each equipped with a polishing pad for performing the CMP operation. A wafer is first loaded into the HCLU station 66 and then the carousel 50 turns 90.degree., 180.degree. and 270.degree. sequentially to perform successive polishing operations on the wafer by the three polishing pads which may be provided in different roughness. At the center of the loading platform 68, an aperture 70 is provided for calibration purpose. Each of the spindle mounted on the spindle mounts 52,54,56 and 58 must be successively calibrated to the load cup 66 prior to the start of a CMP process. During the calibration, a home position is first selected with X, Y parameters stored in a process controller. When a polishing head is loaded with a wafer for carrying out the polishing process, the head must be accurately positioned at the center of the load cup 66 or at the center of any polishing discs (not shown). If the centering is not accurately calibrated, a loading failure occurs which may incorrectly positioned wafer lead to and wafer breakage.
For each of the spindle, a two-step calibration process is required. First, the carousel is rotated 90.degree. each time to calibrate the spindle position in the circumferential direction for its centering in the load cup. After the position in the circumferential direction is calibrated, the traversing platform 60 must also be calibrated in the sweep (or linear) direction, or in the radial direction to center the spindle. The calibration process is difficult and cannot be performed each time with high accuracy.
It is therefore an object of the present invention to provide an apparatus for calibrating the centering of polishing heads in a polishing machine that does not have the drawbacks or shortcomings of the conventional calibration apparatus.
It is another object of the present invention to provide an apparatus for calibrating the centering of polishing heads in a polishing machine equipped with a plurality of spindles for mounting of the polishing heads.
It is a further object of the present invention to provide an apparatus for calibrating the centering of polishing heads in a polishing machine wherein the apparatus is a calibration disk equipped with a calibration pin at its center for alignment with a center aperture in a load cup.
It is another further object of the present invention to provide an apparatus for calibrating the centering of polishing heads in a polishing machine which is equipped with a plurality of spindles for mounting the polishing heads thereto wherein the apparatus is capable of performing calibration in both the circumferential and in the radial direction.
It is still another object of the present invention to provide an apparatus for calibrating the centering of polishing heads in a polishing machine that consists of a calibration disc equipped with a center pin adapted for alignment with a center aperture in a pedestal and a mounting means for mounting the calibration disc to a spindle.
It is yet another object of the present invention to provide an apparatus for calibrating the centering of polishing heads which includes a hollow shaft, a pin that slidingly engaging an elongated cavity in the hollow shaft and a locking device for locking the position of the pin.
It is still another further object of the present invention to provide a method for calibrating the centering of polishing heads in a polishing machine equipped with a plurality of spindles for mounting the polishing heads by utilizing a calibration disc that is equipped with a center pin for alignment with a center aperture in a pedestal.
It is yet another further object of the present invention to provide a method for calibrating the centering of polishing heads in a chemical mechanical polishing machine that is equipped with four spindles for mounting the polishing heads by calibrating the spindles in both the circumferential direction and the radial direction.